The invention relates to a speed responsive power steering for variably controlling the magnitude of a steering power in accordance with a vehicle speed.
A power steering apparatus generally comprises an input shaft which is turned by operating a steering wheel, an output shaft disposed in coaxial relationship with the input shaft and connected thereto through a torsion bar, and a rotary servo valve including an inner valve member integrally rotating with one of the input and the output shaft and an outer valve member integrally rotating with the other shaft. A relative angular displacement between the input and the output shaft switches the servo valve to introduce an operating oil which is discharged from a pump into one of chambers in a power cylinder while returning the operating oil from the other chamber to a tank, thus operating the power cylinder to derive an assisting force.
In the power steering apparatus of the kind described, a greater assisting force is developed to achieve a light steering operation when running at low speeds or when the steering wheel is fixed, while a stable running is ensured by reducing the magnitude of the assisting force by reducing a flow rate supplied to the power cylinder through the servo valve when running at high speeds. FIG. 5 illustrates a circuit diagram of an example of a conventional power steering apparatus where an operating oil discharged from a pump 2 is supplied to or displaced from a power cylinder 6 through a servo valve 4, with a speed responsive valve 16 disposed in parallel relationship with the servo valve 4 so that when the steering wheel is fixed or when the discharge flow from the pump 2 is reduced as during a parking, the speed responsive valve 16 is closed to allow the entire discharge flow from the pump 2 to be supplied to the servo valve 4 to develop a greater asisting force during a steering operation while when running at high speeds, the speed responsive valve 16 is opened to return a part of the discharge flow from the pump to a tank 8, thus reducing the flow rate to the power cylinder 6 which is fed through the servo valve 4 to derive an assisting force of a reduced magnitude.
In the power steering of the prior art as mentioned above, when a steering operation takes place during the time the vehicle is running at a high speed, the servo valve 4 is throttled to cause an increase in its upstream pressure while the flow which is returned to the tank 8 through the opened responsive valve 16 increases to reduce the flow rate which is fed to the power cylinder 6, and accordingly, there is a difficulty in achieving a smooth increase in the oil pressure (see a curve a shown in phantom line in FIG. 6).
To accommodate for this, an arrangement is proposed (Japanese Laid-Open Patent Application No. 306,878/1990 corresponding to U.S. Pat. No. 4,570,736 or WO A1 9014261) in which in addition to a conventional servo valve, which will be hereafter referred to as main servo valve, which supplies or displaces an operating oil to or from a power cylinder, an auxiliary servo valve is disposed in parallel relationship with the main servo valve and is operated together with the main servo valve in response to a relative angular displacement between the input and the output shaft, with a speed responsive valve disposed either upstream or downstream of the auxiliary servo valve.
With this speed responsive power steering apparatus, as the main servo valve is throttled in response to a steering operation which takes place when the vehicle is running at a high speed, the auxiliary servo valve which is operated together with the servo valve is also throttled. This avoids an increase in the return flow which passes through the auxiliary servo valve, and in this manner, the flow rate which is supplied to the power cylinder through the main servo valve is assured, thereby obtaining a desirable response as indicated by a solid-line curve b shown in FIG. 6. However, in such arrangement, because there prevails a high pressure upstream of the main servo valve and the speed responsive valve during a steering operation, the entire discharge flow from the pump represents a dissipation of the engine horsepower, thus presenting a problem of a high power loss. A graph c shown in broken lines in FIG. 6 is included for purpose of comparison, and illustrates the response of a normal power steering which is not provided with the speed responsive valve.